The official, who asked not to be named, said the Iranians had held off on their threat to resume nuclear fuel cycle activities which the United States and its allies fear could be a prelude to efforts to develop a bomb.

Britain, France and Germany, which are trying to lure Iran off its suspected nuclear weapons ambitions with economic and security incentives, have said any resumption of fuel work would scuttle an agreement struck last year in Paris.

But the US official said, "I think there is a general sense that, at least for now, the Iranians have moved away from any immediate action that would break the Paris accord."

Iran announced Monday it was ready to resume the sensitive process of uranium ore conversion that had been suspended for nine months, and was gearing to remove the seals from a plant in the central city of Isfahan.

But it later said it was waiting for inspectors from the International Atomic Energy Agency (IAEA), the UN watchdog, to put surveillance cameras and other controls in place.

The US official took this as providing new leeway for diplomacy. "I think it's extremely positive that the Iranians stepped back from breaking the seals," he said.

"There have been some additional public comments that I've seen, saying, 'Well, we're going to wait for two or three days' and I guess we'll probably wait for a couple of more days beyond that to see the (next) EU-3 proposal."

His assessment contrasted with the more dire view of European officials such as French Foreign Minister Philippe Douste-Blazy who said the parties might be heading towards "a major international crisis."

Both the United States and European Union reiterated Tuesday that a breakdown of negotiations would likely prompt them to refer Iran to the UN Security Council for possible sanctions.

The Europeans were reportedly readying a new offer for Tehran including guaranteed fuel for a nuclear power plant the Iranians are building at Bushehr and others they may construct down the road.

US Undersecretary of State for Political Affairs Nicholas Burns was in daily contact with the Europeans and his boss, Secretary of State Condoleezza Rice, was following developments closely, US officials said.

But the question remained whether Washington and its European allies were in complete agreement on what to offer Tehran. The Americans have insisted Iran completely halt its fuel cycle work and dismantle its facilities.

"I think we're on the same page with the EU-3 in terms of moving forward toward a resolution on this," said acting State Department spokesman Tom Casey.

But he added, "Obviously, I'm not going to be in a position to speak about any proposals that they haven't yet put forward."

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